Fluorescent dyes as functional pigments have been widely used in various fields of science and technology. Especially research on them in the life sciences, clinical diagnostics, immunoassay testing and other aspects has been the world's attention. At present, phenanthridines (EB, PI), acridines (AO), imidazoles (Hoechst, DAPI), cyanine dyes (Cy, TOTO, SYTO) and other commercially available fluorescent dyes have played significant role in the fields of genomics, quantitative detection of nucleic acids, blood cell analysis and etc. However, all these commercially available fluorescent dyes have their own application limitations. Most of them are impermeable and not suitable for live-cell imaging. Only a few of them such as Hoechst and SYTO are suitable for live-cell imaging. Hoechst33258 and Hoechst34580 are the most commonly used probes, but they emit blue fluorescence upon binding to DNA. As the UV light damages cellular DNA, protein and other components, the use of them is very restricted in time [S. K. Davis, C. J. Bardeen, Photochem. Photobiol. 2003, 77, 675-679]. Besides, it's difficult for excitation light to penetrate into the inside biological tissues due to the strong absorption of ultraviolet light by some components in biological samples. Furthermore, the self fluorescence of some biological components may lead to a high fluorescence background that interferes with detection. The class of SYTO compounds show small Stokes shift, which causes the background interference and fluorescence self-quenching. Moreover, due to the traditional cyanine dye structure of SYTO, the defect of their stability can not be ignored. Therefore, exploring novel fluorescent dyes meeting the multiple criteria of excellent spectral properties (e.g. excellent photostability, large Stokes shift), low toxicity and live cell permeability is still the key to promote the development of fluorescent analysis technology, life science and other fields.
Therefore, it is still in great demand to develop of novel fluorescent dyes with the following characteristics: showing low fluorescence background in the absence of nucleic acids, high quantum yield upon binding to nucleic acids and no affinity for biomolecules except for nucleic acids; having excellent photostability, as well as good live cell permeability; exhibiting spectral range with obvious discrepancy from that of biological samples.